Method and Apparatus for Partially Freezing an Aqueous Mixture

ABSTRACT

A method for partially freezing an aqueous mixture comprising simultaneously or in either order the steps of: placing said aqueous mixture in contact with at least part of a freezing surface; cooling the freezing surface to below the freezing point of the aqueous mixture; so that ice forms at the freezing surface; and oscillating the freezing surface relative to the aqueous mixture in a direction that is not perpendicular to at least part of the freezing surface; characterised in that the oscillation is linear with a frequency of between 20 and 200 Hz.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for partially freezing an aqueous mixture to form a dispersion of ice crystals, and hence to produce a water ice slush, frozen milkshake, ice cream or similar product.

BACKGROUND

Conventionally, ice cream or water ice slushes are produced by placing a mixture, which is an aqueous solution and / or suspension of the ingredients, in contact with a freezing surface. A layer of ice forms on the surface. In order to prevent the formation of very large ice crystals and to maintain good heat transfer between the freezing surface and the mixture, it is necessary continuously to remove the layer of ice. This has been achieved by mechanically scraping the ice layer from the freezing surface, for example in factory ice cream freezers (also known as scraped surface heat exchangers), domestic ice cream freezers, and in water ice slush machines such as those used in cafes and convenience stores. This requires the provision of a scraping mechanism and a driving mechanism. These features significantly add to the size and complexity of the production equipment. Furthermore scraping imposes a substantial energy cost, via the friction on the scraping device which originates from overcoming the adhesion force of the ice to the surface.

EP0584127 describes an alternative means of de-icing a freezing surface using high frequency sound waves (ultrasound). Ultrasound is believed to set up a resonance in the freezing surface causing it to flex and bend thereby detaching the ice. This allows ice generation without scraping and has the advantage of simplifying the equipment since it does require moving parts. However, this method has several drawbacks. Engineering the freezing surface to allow ultrasonic resonance presents severe restrictions on the size and design of the device. Also, the use of ultrasound can lead to unpleasant high-pitched noise, which may not be acceptable in a factory or public environment.

One of the big problems faced when making water ice slushes, ice creams and frozen milk shakes is therefore to prevent the build up of ice on the freezing surface without resorting to complex or expensive equipment. Thus there is a need for a simple method of removing ice from freezing surfaces.

It has now been discovered that it is possible to release ice from a freezing surface which is in contact with an aqueous mixture by linearly oscillating the freezing surface at low frequencies.

Tests and Definitions

Partial freezing of an aqueous mixture means freezing the mixture such that only part of the water in the mixture is converted into ice crystals, such that the partially frozen product is suitable for pumping, moulding, extruding, pouring, drinking, spooning and the like.

The freezing point of the aqueous mixture means the equilibrium freezing point of a solution of the initial concentration. The equilibrium freezing point of a solution is lower than the equilibrium freezing point of the pure solvent due to freezing point depression. When a solution is partially frozen, the concentration of the solution increases because water is removed in the form of pure ice crystals (this is known as freeze concentration). Therefore the freezing point decreases further. The freezing point of an aqueous mixture can be determined by methods well known to those skilled in the art.

The terms “milk shake”, “water ice” and “ice cream” have the meanings as stated in Chapter 1 of Ice Cream 4^(th) Edition—W. S. Arbuckle—AVI Publishing, 1986, except that in the context of the present invention, “ice cream” also encompasses compositions comprising vegetable fats.

The angle between the direction of oscillation and the freezing surface at any particular point on the surface means (90—theta) where theta is the smaller of the two angles between the vector normal to the surface at that point and the oscillation direction, as shown in FIG. 1.

Measurement of the Temperature of the Freezing Surface

The temperature of the freezing surface is measured by attaching a self-adhesive thermocouple (T-type, Omega Engineering Ltd, 1 Omega Drive, Riverbound Technology Centre, Northbank, Irlam, Manchester, M44 5BD, UK) to the freezing surface.

Release of Ice From the Freezing Surface

Release of ice from the freezing surface is assessed by visual inspection of the freezing surface. Ice release is judged to be successful when no ice adheres to the surface at the end of the freezing process.

Estimation of Ice Content

The concentration of a solution in equilibrium at a temperature between its freezing point and its glass transition temperature is given by the freezing point curve on the phase diagram. (The phase diagram for sucrose solutions can be found in S. Ablett, M. J. Izzard, P. J. Lillford J. Chem Soc Faraday Trans. 88 (1992) 789). For example, at −2.2° C., the sucrose concentration in equilibrium with ice is 22% (w/w). If the initial concentration of the solution was 20% w/w then the amount of ice formed can be estimated as follows.

Sucrose concentration (% w/w)

-   -   =mass of sucrose / mass of solution     -   =mass sucrose/(total mass−mass of ice)     -   22%=20/(100−x)*100         where x=% (w/w) ice=9.8% in this example.

BRIEF DESCRIPTION OF THE INVENTION

It is the first object of the present invention to provide a method for partially freezing an aqueous mixture comprising simultaneously or in either order the steps of: placing said aqueous mixture in contact with at least part of a freezing surface, cooling said freezing surface to below the freezing point of said aqueous mixture, so that ice forms at the freezing surface; and oscillating said freezing surface relative to said aqueous mixture in a direction that is not perpendicular to at least part of the freezing surface, characterised in that the oscillation is linear with a frequency of between 20 and 200 Hz.

Preferably the frequency of the oscillation is between 40 Hz and 100 Hz. More preferably the frequency of the oscillation is between 50 Hz and 80 Hz. It has been found that the higher the frequency of the oscillation within these ranges, the better is the removal of ice from the freezing surface.

Preferably the amplitude of the oscillation is between 0.2 mm and 20 mm. More preferably the amplitude of the oscillation is between 1 mm and 15 mm. Most preferably the amplitude of the oscillation is between 4 mm and 10 mm. It has been found that the larger the amplitude of the oscillation within these ranges, the better is the removal of ice from the freezing surface.

The oscillation may be of any suitable waveform, for example sinusoidal, square wave or saw tooth. Preferably the oscillation is sinusoidal.

It is desirable that the rate of partial freezing of the aqueous mixture should be as rapid as possible, so that the rate of production of the partially frozen product, such as water ice slush, ice cream or frozen milkshake, is maximized. It is believed that the rate of partial freezing depends on (at least) three factors: the rate of formation of ice at the freezing surface, the area of the freezing surface and the rate of release of ice from the freezing surface.

It has been found that the more closely the freezing surface is parallel to the direction of oscillation, the faster the release of ice from the freezing surface. The shape of the freezing surface is therefore chosen so that the angle between the direction of oscillation and the majority of the freezing surface is less than 45°. For example the freezing surface comprises the surface of a rod with its axis is parallel to the direction of oscillation. Preferably the freezing surface is the surface of a cylinder with its axis parallel to the direction of oscillation. More preferably, the cylinder is vertical and the lower end of the cylinder comprises a protrusion. Most preferably the protrusion is a hemisphere or a cone.

It has been found that the larger the area of the freezing surface, the more ice is generated. The shape of the freezing surface is chosen to have a large surface area. Preferably, the freezing surface comprises the inner and outer surfaces of a cylindrical tube with its axis parallel to the direction of oscillation.

The freezing surface can comprise the surfaces of a plurality of members which are rigidly mounted onto a single base. Preferably the members comprise cylinders with their axes parallel to the direction of oscillation. More preferably the cylinders are vertical and lower end of the cylinder comprises a protrusion. Most preferably the protrusion is a hemisphere or a cone. Alternatively the members comprise cylindrical tubes with their axes parallel to the direction of oscillation.

The area of the freezing surface can be increased by the addition of fins. The fins may be flat plates or may be shaped, for example twisted, to enhance axial mixing in the aqueous mixture. Preferably one or more fins are attached to the freezing surface.

The strength of the adhesion of the ice to the freezing surface depends on the temperature of the freezing surface, and also on the type of solute in the aqueous mixture. It has been found that for temperatures in the range of 0° C. to −20° C., the lower the temperature of the freezing surface, the stronger the adhesion. However, the higher the temperature of the freezing surface, the slower the rate of formation of ice (The temperature of the freezing surface must be below the freezing point of the aqueous mixture in order to form ice.). Thus the optimal temperature of the freezing surface for a particular aqueous mixture is determined by a compromise between these two opposing effects. It has been found that a rapid rate of ice formation and easy release is achieved when the temperature of the freezing surface is between −1° C. and −20° C. Preferably the temperature of the freezing surface is below −5° C. Equally preferably the temperature of the freezing surface is above −10° C. Most preferably the temperature of the freezing surface is between −5° C. and −10° C.

It has further been found that the ice removal can be enhanced without significantly reducing the rate of ice formation by cycling the temperature of the freezing surface from a temperature between 5° C. and 25° C. below the freezing point of the aqueous mixture to a temperature more than 0° C. and less than 5° C. below the freezing point of the aqueous mixture.

The freezing surface is cooled by any suitable cooling means, for example by flowing a coolant, such as aqueous ethylene glycol or Freon, through the interior of the member whose surface comprises the freezing surface.

It has been found that the method of the present invention can be used to produce partially frozen foods or drinks when a suitable aqueous mixture is used. Preferably the aqueous mixture comprises an aqueous solution and/or suspension of edible ingredients selected from the group consisting of sugars, food acids, colours, flavours, proteins, fats emulsifiers and stabilisers. More preferably the aqueous mixture is a milk shake, water ice mix or an ice cream mix.

It is a second object of the invention to provide an apparatus for partially freezing an aqueous mixture comprising a freezing surface, a cooling means capable of cooling said freezing surface to below −1° C., and an oscillation means coupled to said freezing surface characterised in that said oscillating means is capable of linearly oscillating said freezing surface relative in a direction that is not perpendicular to at least part of the freezing surface with a frequency of between 20 and 200 Hz.

Preferably the oscillation means is capable of oscillating the freezing surface with an amplitude of between 1 mm and 20 mm.

Preferably the cooling means is capable of cooling the freezing surface to below −5° C., more preferably to below −10° C.

It is desirable that the apparatus should be simple and inexpensive. Preferably the oscillation means is a loud speaker, a magnetic coil, an electrodynamic shaker or a reciprocating electric motor.

The oscillation means may be coupled to the freezing surface by direct coupling, or by a resilient member, or by a cantilever beam. It has been found that direct coupling provides a simple, inexpensive means of coupling. It has been further found that a cantilever beam is suitable for oscillating heavy freezing surfaces. It has also been found that by coupling the freezing surface to the oscillation means with a resilient member, such as a flexible beam or a spring, and oscillating at its resonant frequency, large amplitude oscillations can be obtained.

The shape of the freezing surface is chosen so that the angle between the direction of oscillation and the majority of the freezing surface is less than 45°. For example the freezing surface comprises the surface of a rod with its axis is parallel to the direction of oscillation. Preferably the freezing surface is the surface of a cylinder with its axis parallel to the direction of oscillation. More preferably, the cylinder is vertical and the lower end of the cylinder comprises a protrusion. Most preferably the protrusion is a hemisphere or a cone.

The freezing surface can comprise the surfaces of a plurality of members which are rigidly mounted onto a single base which is oscillated by a single oscillation means. This avoids the necessity for more than one oscillation means. Preferably the members comprise cylinders with their axes parallel to the direction of oscillation. More preferably the cylinders are vertical and lower end of the cylinder comprises a protrusion. Most preferably the protrusion is a hemisphere or a cone. Alternatively the members comprise cylindrical tubes with their axes parallel to the direction of oscillation.

DETAILED DESCRIPTION

The present invention will be further described by reference to the drawings, wherein;

FIG. 1 illustrates the definition of the angle between the direction of oscillation and the freezing surface.

FIG. 2 represents a schematic view of the apparatus according to the second aspect of the invention, together with an aqueous mixture.

FIG. 3 represents a freezing surface in accordance with the invention, comprising a cylinder with a hemispherical protrusion on its lower end.

FIG. 4 represents a freezing surface in accordance with the invention, comprising a cylindrical tube.

FIG. 5 represents a freezing surface in accordance with the invention, to which fins are attached.

FIG. 6 represents a freezing surface in accordance with the invention, comprising a plurality of members rigidly mounted on a single base which is oscillated by a single driving mechanism.

FIG. 7 represents a detailed diagram of the apparatus in accordance with the second aspect of the invention wherein the oscillation means is a loudspeaker with a resilient beam coupling.

FIG. 1 shows freezing surface 3, the vector 30 normal to a point on the freezing surface 3 and the direction of oscillation 5. The angle 31 is the smaller of the two angles between the vector 30 normal to the surface and the oscillation direction 5. The angle between the direction of oscillation and the freezing surface is given by (90°—angle 31).

FIG. 2 represents a schematic view of the apparatus, together with an aqueous mixture. In FIG. 2 the oscillation means 1 is coupled to the freezing surface 3 by means of a coupling 2. The freezing surface 3 is immersed in an aqueous mixture 4. The oscillation means 1 oscillates freezing surface 3 in direction 5.

FIG. 3 represents a freezing surface 3 comprising a cylinder 6 with a hemispherical protrusion 7 on its lower end. The temperature of freezing surface 3 is controlled by flowing a coolant liquid 8 through cylinder 6 via inlet 9 and outlet 10.

FIG. 4 shows a cross-sectional view of a cylindrical tube 11. The tube has outer surface 12 and inner surface 13 which together comprise the freezing surface. The tube is hollow to allow the freezing surface to be cooled with coolant liquid 8.

FIG. 5 shows a top view of a freezing surface 3 to which fins 14 are attached. They may consist of flat plates or may be shaped so as to enhance mixing of the aqueous mixture.

FIG. 6 represents a freezing surface comprising a plurality of members 16 rigidly mounted on a single base 15 which can be oscillated in direction 5 by a single oscillation means.

FIG. 7 shows an oscillation means consisting of a loudspeaker (with its speaker cone removed) comprising a magnet 17, pole pieces 18, coil 19 and frame 20. A lightweight tube 21 is attached to the tube 22 around which the coil is wrapped. A linear bearing 23 provides axial alignment for the tube 21. The tube is coupled to the freezing surface 3 by means of a resilient beam 25 and rod 26. The beam is supported at both ends on knife edges 24 and the freezing surface 3 is attached to the centre of the beam.

The present invention will be further described with reference to the following examples which are illustrative only and non-limiting.

EXAMPLE 1 Freezing Surfaces of Various Shapes

(a) A vertical hollow copper cylinder with length 90 mm, diameter 16 mm and wall thickness 1 mm was directly coupled at its upper end to an electrodynamic shaker (model V406, Ling Dynamic Systems Ltd, Royston, Herts, UK). The lower end of the cylinder was closed by a flat plate. An aqueous solution of 50% w/w ethylene glycol at −20° C. was passed through the interior of the cylinder by means of a Haake refrigerated circulator. The cylinder was sinusoidally oscillated along its longitudinal axis at a frequency of 60 Hz and an amplitude of 6 mm. A cup containing 250 ml of a 20% w/w sucrose solution, initially at a temperature close to 0° C., was positioned under the cylinder so that the cooled cylinder was fully immersed in the solution. Ice continually formed on the curved surface of the cylinder and was released into the solution by the oscillation. Ice also formed on the flat end of the cylinder, but was not removed and continued to build up over time.

(b) An identical cylinder was constructed except that it had a hemispherical protrusion at its free end rather than a flat plate. Use of this cylinder under the same conditions resulted in successful release of ice from the entire surface and no build-up at the free end.

(c) A third freezing surface consisting of a cylindrical tube with length 115 mm, outer diameter 42 mm, inner diameter 34 mm and wall thickness 0.9 mm was constructed from aluminium. The surface was polished. The mass of the cylinder when empty was 118 g. The tube was cooled by flowing coolant through the walls. Use of this cylinder under the same conditions resulted in the formation of patches of ice on the inner and outer surfaces of the tube. The ice was released into the surrounding solution by the oscillation.

EXAMPLE 2 Amplitude and Frequency of Oscillation

Without wishing to be limited by theory, it is believed that the ice is removed from the surface by the shear force between the ice layer on the freezing surface and the aqueous mixture. This is related to the maximum acceleration of the surface during the oscillation in the direction parallel to the surface. The larger the maximum acceleration, the greater the ice removal force. Increasing both the amplitude and frequency of the oscillation increases the maximum acceleration. The amplitude and frequency of the oscillation required to remove the ice from the freezing surface depends on the strength of the adhesion of the ice, which in turn depends on the temperature of the surface and the nature and concentration of the solutes in the aqueous mixture. Increasing the frequency or the amplitude of the oscillation has been found to increase the de-icing ability of the freezing surface.

The cylindrical tube was used as described in Example 1 (c) to partially freeze a 20% w/w sucrose solution using a 50% w/w ethylene glycol at −20° C. as the coolant. The frequency and amplitude of oscillation were varied. At each frequency the minimum amplitude required to release ice from the freezing surface was as follows: 20% sucrose, −20° C. Frequency (Hz) Amplitude (mm) 150 ≧1.5 100 ≧2 80 ≧2.5 60 ≧3 40 ≧5

Thus at any given frequency, ice release can be achieved when the amplitude of the oscillation is increased above a certain value.

EXAMPLE 3 Effect of Solution Concentration

The experiment of example 2 was repeated using a 30% w/w sucrose solution. Ice release was achieved under the following conditions: 30% sucrose, −20° C. Frequency (Hz) Amplitude (mm) 150 ≧1 100 ≧1.5 80 ≧2 60 ≧2.5 40 ≧4

Thus it can be seen by comparing Examples 2 and 3.that the amplitude and frequency required for ice release depend on the concentration of the solution. Increasing the sucrose concentration from 20 to 30% w/w reduced the amplitude required for release at any given frequency.

EXAMPLE 4 Effect of Coolant Temperature

The experiment of example 2 was repeated using an ethylene glycol solution at −10° C. Ice release was achieved under the following conditions: 20% sucrose, −10° C. Frequency (Hz) Amplitude (mm) 150 ≧0.5 100 ≧1 60 ≧1.5 40 ≧3 20 ≧5

Thus it can be seen by comparing Examples 2 and 4 that the amplitude and frequency required for ice release depend on the temperature of the freezing surface (which depends on the temperature of the coolant). Increasing the temperature of the coolant sucrose concentration from −10 to −20° C. reduced the amplitude required for release at any given frequency.

EXAMPLE 5 Effect of Cycling the Temperature of the Freezing Surface

The experiment of example 2 was repeated but with the flow of ethylene glycol through the finger stopped periodically (20 s on, 20 s off) causing the temperature of the freezing surface to rise and fall periodically. The temperature was measured as specified above using a thermocouple placed on the outside surface of the cylinder, approximately 40 mm from the upper end and close to the coolant inlet port.

The ice was released more easily when the surface temperature was periodically cycled. This is due to the lower adhesion at higher surface temperatures. This enabled a 10% sucrose solution to be partially frozen using the amplitude and frequency corresponding to that which enabled a 20% sucrose solution to be partially frozen without temperature cycling.

EXAMPLE 6 Effect of Changing Solute

The experiment of example 2 was repeated using an 8% glycerol solution instead of 20% sucrose solution. Ice release was obtained under the following conditions: 8% sucrose, −20° C. Frequency (Hz) Amplitude (mm) 150 ≧1 100 ≧2 80 ≧2.5 60 ≧3 40 ≧5

Thus it can be seen by comparing Examples 2 and 6 that the amplitude and frequency required for ice release depend on the nature of the solute as well as the concentration of the solution. Ice release occurs with approximately the same conditions for 8% w/w glycerol as for 20% w/w sucrose.

EXAMPLE 7 Ice Content After 2 Minutes Freezing

The experiment of example 2 was repeated. The cylinder was placed in the 20% sucrose solution and oscillated at a frequency of 60 Hz. Three experiments were performed with different amplitudes. In each case the cylinder was placed in the solution for a period of 2 minutes, after which the partially frozen mixture was gently stirred and its temperature was measured using a Comark temperature probe. The ice content was estimated from the temperature using the method described above and the results were as follows. Amplitude (mm) Temperature (° C.) Ice content (% w/w) 4 −1.6 5 6 −1.7 10 8 −1.8 13

Higher ice contents were achieved at higher displacements because the more effective de-icing process allowed higher rates of heat transfer to be achieved.

EXAMPLE 8 Production of a Water Ice Slush

A mixture was prepared with the following composition: (% w/w) Sucrose 10 Dextrose monohydrate 6 63 DE low fructose corn syrup 6 Citric acid 0.6 Potassium sorbate 0.03 Lemon and lime flavour 0.03 Water to 100

This was partially frozen using the set-up described in example 2 with an oscillation frequency of 60 Hz and an amplitude of 8 mm. The temperature was measured after 2 minutes to be −2.3° C. The resulting product was judged to contain sufficient ice and to be an acceptable slush ice drink.

EXAMPLE 9 Production of an Ice Cream

A basic ice cream mix was prepared with the following composition: (% w/w) Sucrose 20 Skim milk powder 10 Milk fat 10 Water to 100

This was frozen using the set-up described in example 2 with an oscillation frequency of 60 Hz and an amplitude of 8 mm. The temperature was measured after 2 minutes to be −4.9° C. The resulting product was judged to contain sufficient ice and to be an acceptable soft ice cream.

EXAMPLE 10 Cantilever Beam Coupling

A larger freezing surface consisting of a cylindrical tube was constructed from copper. The mass of the cylinder when empty was approximately 1 kg. The directly coupled electrodynamic shaker was not capable of oscillating the heavy tube with sufficiently large amplitude. Instead the tube was coupled to the shaker by a steel cantilever beam (255 mm×75 mm×10 mm). One end of the beam was clamped to a large block of steel (the fixed end) and the other was attached to the tube (the free end). The beam was driven by a pushrod attached to the shaker between the fixed end and the free end. The system was tuned to resonance by sweeping the oscillation frequency until maximum amplitude was obtained. The resonant frequency depends on the length of the beam, so the beam was chosen such that its first bending resonant frequency was the chosen operating frequency (50 Hz). Ice release was obtained in a 20% sucrose solution with using an ethylene glycol solution at −10° C. as the coolant at amplitudes of 2.4 mm and above.

EXAMPLE 11 Loudspeaker with a Resilient Beam Coupling

An alternative oscillation means was constructed from a 100 W loudspeaker with the speaker cone removed. A lightweight tube was attached to the tube around which the coil is wrapped. A linear bearing was provided to provide axial alignment for the tube. The linear bearing consisted of two perspex plates spaced 20 mm vertically apart with concentric holes through which the tube could slide. The tube was coupled to the freezing surface which consisted of a copper cylinder (diameter 22 mm, length 110 mm) by means of a resilient beam and rod. The beam was supported at both ends on knife edges and the freezing surface was attached to the centre of the beam. The beam was chosen so that its resonant frequency matched the operating frequency (50 Hz). Large oscillation amplitudes (>10 mm) could be achieved with this arrangement.

The various features of the embodiments of the present invention referred to in individual sections above apply, as appropriate, to other sections mutatis mutandis. Consequently features specified in one section may be combined with features specified in other sections as appropriate.

All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and products of the invention will be apparent to those skilled in the art without departing from the scope of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are apparent to those skilled in the relevant fields are intended to be within the scope of the following claims. 

1. A method for partially freezing an aqueous mixture simultaneously or in either order the steps of: placing said aqueous mixtures in contact with at least part of a freezing surface; cooling said freezing surface to below the freezing point of said aqueous mixture; so that ice forms at the freezing surface; and oscillating said freezing surface relative to said aqueous mixture in a direction that is not perpendicular to at least part of the freezing surface; characterized in that the oscillation is linear with a frequency of between 20 and 200 Hz.
 2. A method according to claim 1 wherein the frequency of the oscillation is between 40 and 100 Hz.
 3. A method according to claim 1 wherein the amplitude of the oscillation is between 0.2 mm and 20 mm.
 4. A method according to claim 1 wherein the amplitude of the oscillation is between 4 mm and 10 mm.
 5. A method according to claim 1 wherein the angle between the direction of oscillation and the majority of the freezing surface is less than 45°.
 6. A method according to claim 1 wherein the freezing surface is the surface of a cylinder with its axis parallel to the direction of oscillation.
 7. A method according to claim 6 wherein the freezing surface is the surface of a vertical cylinder the lower end of which comprises a hemispherical protrusion.
 8. A method according to claim 1 wherein the freezing surface comprises the inner and outer surfaces of a cylindrical tube with its axis parallel to the direction of oscillation.
 9. A method according to claim 1 wherein the freezing surface comprises the surfaces of a plurality of members which are rigidly mounted onto a single base.
 10. A method according to claim 9 wherein the members are cylinders or cylindrical tubes with their axes parallel to the direction of oscillation.
 11. A method according to claim 1 in which the temperature of the freezing surface is between −1° C. and −20° C.
 12. A method according to claim 1 in which the temperature-of the freezing surface is cycled from a temperature between 5° C. and 25° C. below the freezing point of the aqueous mixtures to a temperature more than 0° C. and less than 5° C. below the freezing point of the aqueous mixture.
 13. A method according to claim 1 wherein the aqueous mixture comprises an aqueous solution and/or suspension of edible ingredients selected from the group consisting of sugars, food acids, colours, flavours, proteins, fats emulsifiers and stabilizers.
 14. A method according to claim 13 in which the aqueous mixture is a milk shake, water ice mix or an ice cream mix.
 15. An apparatus for partially freezing an aqueous mixtures comprising: a freezing surface; a cooling means capable of cooling said freezing surface to below −1° C. and an oscillation means which is coupled to said freezing surface characterized in that said oscillating means is capable of linearly oscillating said freezing surface in a direction that is not perpendicular to at least part of the freezing surface with a frequency of between 20 and 200 Hz.
 16. An apparatus according to claim 15 wherein the oscillation means is capable of oscillating said freezing surface with an amplitude of between 1 mm and 20 mm.
 17. An apparatus according to claim 15 wherein the oscillation means is selected from the group consisting of a loud speaker, a magnetic coil, an electrodynamic shaker and a reciprocating electric motor.
 18. An apparatus according to claim 15 wherein the oscillation means is coupled to the freezing surface by a coupling means selected from the group consisting of direct coupling, a resilient member, and a cantilever beam.
 19. An apparatus according to claim 15 wherein the freezing surface comprises the surfaces of a plurality of members rigidly mounted onto a single base and which are capable of being oscillated by a single oscillation means. 